As noted above, header fields and property values may be NULL. The evaluation of selector expressions containing NULL values is defined by SQL 92 NULL semantics. A brief description of these semantics is provided here. SQL treats a NULL value as unknown. Comparison or arithmetic with an unknown value always yields an unknown value. The IS NULL and IS NOT NULL operators convert an unknown header or property value into the respective TRUE and FALSE values. The boolean operators use three-valued logic as defined by the following tables: Table 3-4 The definition of the AND operator AND T F U T T F U F F F F U U F U